I encountered a weird bug with deserializing JSON in a JRuby app during an OpenJDK upgrade - it would sporadically throw a parse error for no apparent reason. I was upgrading to OpenJDK 15, but another user experienced the same regression with an LTS upgrade from 8 to 11.

The end result of my own investigation led to this quite satisfying thread on hotspot-compiler-dev, in which an engineer starts with my minimal reproduction of the problem and posts a workaround within 24 hours: https://mail.openjdk.org/pipermail/hotspot-compiler-dev/2021...

There's also a tip there: try a fastdebug build and see if you can convert it into an assertion failure you can look up.